Use the Maven Wrapper in the Gitea workflow and check the registry token

act-based Gitea runner images do not ship Maven, so the publish job failed
with 'mvn: command not found'. Add the Maven Wrapper (Maven 3.9.11) and use
./mvnw. Fail early with a clear message when REGISTRY_TOKEN is not available.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

README.md

@@ -4,10 +4,10 @@ OpenPGP encryption for Spring Boot mail: discovers recipients' keys via Web Key
 PGP/MIME (RFC 3156), or hands mail to an encrypting relay. See [docs/PLAN.md](docs/PLAN.md) for the design
 and milestone status.
 
-Requires JDK 27 and Maven 3.9+. Built on Spring Boot 4.1.1 and PGPainless 2.0.
+Requires JDK 27; the Maven Wrapper (`./mvnw`) downloads Maven 3.9.11. Built on Spring Boot 4.1.1 and PGPainless 2.0.
 
 ```
-mvn verify
+./mvnw verify
 ```
 
 ## Modules
@@ -82,7 +82,7 @@ the outcome per recipient: PGP/MIME-encrypted (RFC 3156) when a valid WKD key is
 Set `MAIL_PGP_MODE=RELAY` and `RELAY_HOST`/`RELAY_PORT`/`RELAY_USERNAME`/`RELAY_PASSWORD` to send via a gateway.
 
 ```
-mvn -q install -DskipTests
+./mvnw -q install -DskipTests
 SMTP_HOST=smtp.example.com SMTP_PORT=587 \
 SMTP_USERNAME=me@example.com SMTP_PASSWORD='app-password' \
 DEMO_TO=test@thinhha.pro DEMO_COUNT=1 \